1. Field of the Invention
The present invention relates generally to image forming apparatuses, and more particularly to an image forming apparatus, such as a printer, of an electrophotography type that performs misregistration correction in producing a visualized image by superposing multiple colors.
2. Description of the Related Art
In conventional image forming apparatuses, image forming parts of respective colors are arranged along a belt conveyor (endless moving part) as shown in FIG. 1. This is called a tandem type. That is, multiple image forming parts (electrophotographic process parts) 6Y, 6M, 6C, and 6BK are arranged along a belt conveyor 5 in this order from the upstream side in the direction in which the belt conveyor 5 conveys sheets of paper (recording paper) 4 separated and fed one by one from a paper feed tray 1 by a paper feed roller 2 and separation rollers 3. These image forming parts 6Y, 6M, 6C, and 6BK are different only in the color of a formed toner image and are equal in internal configuration. The image forming part 6Y forms a yellow image, the image forming part 6M forms a magenta image, the image forming part 6C forms a cyan image, and the image forming part 6BK forms a black image.
A specific description is given of the image forming part 6Y. The other image forming parts 6M, 6C, and 6BK have the same configuration as the image forming part 6Y. Accordingly, the elements of each of the image forming parts 6M, 6C, and 6BK are referred to in the drawing by the same reference numerals as those of the corresponding elements of the image forming part 6Y except that the suffix Y is replaced with the suffix of the corresponding color M, C, or BK, and a description thereof is omitted.
The belt conveyor 5 is an endless belt wound around a driving roller 7 and a driven roller 8. The driving roller 7 is rotated by a drive motor (not graphically illustrated). This drive motor, the driving roller 7, and the driven roller 8 serve as a driving part that moves the belt conveyor 5 that is an endless moving part.
In image formation, the sheets of paper (paper sheets) 4 contained in the paper feed tray 1 are fed in order starting from the uppermost paper sheet. The fed paper sheet 4 is attracted and adhered to the belt conveyor 5 by electrostatic adhesion, and conveyed by the rotating belt conveyor 5 to the first image forming part 6Y, where a yellow toner image is transferred onto the paper sheet 4.
The image forming part 6Y includes a photosensitive body drum 9Y serving as a photosensitive body, and a charger 10Y, an exposure unit 11, a developing unit 12Y, a photosensitive body cleaner (not graphically illustrated), and a discharger 13Y disposed around the photosensitive body drum 9Y. The exposure unit 11 is configured to emit laser lights 14Y, 14M, 14C, and 14BK that are exposure lights corresponding to the colors of images formed by the image forming parts 6Y, 6M, 6C, and 6BK, respectively.
In image formation, after being evenly charged by the charger 10Y in the dark, the exterior (cylindrical) surface of the photosensitive body drum 9Y is exposed to the laser light 14Y corresponding to a yellow toner image from the exposure unit 11, so as to have an electrostatic latent image formed thereon. The developing unit 12Y visualizes this electrostatic latent image with yellow toner, so that the yellow toner image is formed on the photosensitive body drum 9Y. This toner image is transferred onto the paper sheet 4 through the action of a transfer unit 15Y at a transfer position, that is, a position where the photosensitive body drum 9Y and the paper sheet 4 on the belt conveyor 5 come into contact with each other. As a result of this transfer, the yellow toner image is formed on the paper sheet 4. After the transfer of the toner image is completed, unnecessary toner remaining on the external surface of the photosensitive body drum 9Y is wiped off (removed) by the photosensitive body cleaner. Thereafter, the photosensitive body drum 9Y is discharged by the discharger 13Y, and stands by for the next image forming operation.
The paper sheet 4 having the yellow toner image transferred thereonto in the image forming part 6Y in the above-described manner is conveyed to the next image forming part 6M by the belt conveyor 5. In the image forming part 6M, a magenta toner image is formed on a photosensitive body drum 9M by the same image forming process as the image forming process in the image forming part 6Y, and the magenta toner image is transferred so as to be superposed on the yellow toner image formed on the paper sheet 4.
The paper sheet 4 is further conveyed to the next image forming parts 6C and 6BK, so that a cyan toner image formed on a photosensitive body drum 9C and a black toner image formed on a photosensitive body drum 9BK are transferred onto the paper sheet 4 in a superposed manner by the same operations. As a result, a full color image is formed on the paper sheet 4. The paper sheet 4 having this full color composite image formed thereon is separated from the belt conveyor 5 and has the full color image fixed thereonto in a fuser 16. Thereafter, the paper sheet 4 is ejected outside the image forming apparatus.
In the color image forming apparatus having the above-described configuration, there may be a problem in that the toner images of respective colors are not superposed where they are supposed to be because of errors such as center distance errors of the photosensitive body drums 9Y, 9M, 9C, and 9BK, parallelism errors of the photosensitive body drums 9Y, 9M, 9C, and 9BK, provision errors of deflection mirrors (not graphically illustrated) that deflect the laser lights 14Y, 14M, 14C, and 14BK in the exposure unit 11, and timing errors in writing electrostatic latent images onto the photosensitive body drums 9Y, 9M, 9C, and 9BK, so that misregistration may be caused between colors. Skew, misregistration in the sub scanning direction, magnification error in the main scanning direction, and misregistration in the main scanning direction are known as principal components of color misregistration. Therefore, it is necessary to correct misregistration (misalignment) of each color toner image. As shown in FIG. 1, sensors 17, 18, and 19 are provided on the downstream side of the image forming part 6BK so as to face the belt conveyor 5. The sensors 17, 18, and 19 are supported by the same substrate so as to be along the main scanning direction perpendicular to the direction in which the paper sheet 4 is conveyed.
FIG. 2A is a diagram showing image detecting (sensing) parts (including the sensors 17, 18, and 19) and their periphery. FIG. 2B is an enlarged view of one of the image detecting parts.
Referring to FIG. 2A, the image detecting parts are provided at both ends and in the center in the main scanning direction, and marks for detecting misregistration (misregistration detection marks) 23 (23a, 23b, and 23c) are formed on the belt conveyor 5 for the corresponding image detecting parts.
Referring to FIG. 2B, each image detecting part includes a light emitting part 20, a slit 21, and a light receiving part 22, and detects (senses) the corresponding misregistration detection mark 23 formed on the belt conveyor 5.
FIG. 2C is an enlarged view of the slit 21. In order to detect a line parallel to the main scanning direction (parallel line) and a line inclined with respect to the parallel line (inclined line), the slit 21 includes openings parallel to the parallel line and the inclined line.
FIG. 3A is an enlarged view of the misregistration detection marks 23. Each misregistration mark 23 includes parallel and inclined lines of each of K (BK), M, Y, and C. The parallel and inclined lines are formed with a target of a predetermined distance d between all adjacent parallel lines and between all adjacent inclined lines. As a result, a detection signal has a well-shaped peak or valley waveform when the line reaches the corresponding opening of the slit 21, so that it is possible to determine the center of the line with accuracy.
FIG. 3B shows a configuration for processing data thus detected. A CPU 31 performs a predetermined operation based on the results of detection of the misregistration detection marks 23, so that the amounts of skew, misregistration in the sub scanning direction, magnification error in the main scanning direction, and misregistration in the main scanning direction are determined. Correction is performed based on these results. With respect to skew, for example, an inclination is added to a deflection mirror in the exposure unit 11 or to the exposure unit 11 itself through an actuator. With respect to misregistration in the sub scanning direction, correction is performed, for example, by controlling writing start timing of lines and the surface phase of a polygon mirror. With respect to magnification error in the main scanning direction, correction is performed, for example, by changing a writing image frequency. With respect to misregistration in the main scanning direction, correction is performed by correcting writing start timing of main scanning lines.
FIG. 3A shows a minimum set of mark columns required to determine various amounts of color misregistration of each color. Alternatively, in order to cancel variation error due to variations in the rotations of a photosensitive body, an intermediate transfer belt, and/or a belt conveyor, for example, multiple sets of mark groups may be formed with respect to the period of one rotation of the photosensitive body. The sensors 17, 18, and 19 may detect the mark groups, and the detection results may be averaged. As a result, it is possible to perform detection with more accuracy.
Further, a description is given, with reference to FIG. 3B, of an operation for processing the detected data. A signal obtained from the light-receiving part 22 is amplified by an amplifier (AMP) 24, and only its signal component of line detection is transmitted through a filter 25 so as to be converted from analog data into digital data by an analog-to-digital (A/D) converter part 26. The sampling of data is controlled by a sampling control part 27, and the sampled data are stored in a FIFO memory 28. After detection of one set of misregistration detection marks 23 is completed, the stored data are loaded into the CPU 31 and a RAM 32 through an I/O port 29 and a data bus 30, and the CPU 31 performs a predetermined operation on the data to determine the above-described various amounts of misregistration.
A ROM 33 contains various programs for controlling a misregistration correction unit and the image forming apparatus, such as programs for calculating the above-described various amounts of misregistration. Further, the CPU 31 is properly timed to monitor the detection signal from the light-receiving part 22 so that the amount of light emission is controlled by a light emission amount control part 34 so as to ensure the detection even if the belt conveyor 5 or the light emission part 20 is degraded, thereby causing the level of the light reception signal from the light-receiving part 22 to be always constant. Thus, the CPU 31 and the ROM 33 serve as a control part to control the operation of the entire image forming apparatus. Examples of the conventional techniques related to this are shown below.
According to Patent Document 1 listed below, a disclosed color image forming apparatus prevents misregistration from remaining after correcting misregistration of each color. Toner images of respective colors are formed by an image process part. The color toner images are superposed on a transfer belt so as to form a color image to be transferred (transfer color image). The transfer color image is transferred onto transfer paper. Correction patterns formed on the transfer belt are detected and recognized so as to calculate a correction value. In order to prevent misregistration of each color, this correction value is added in controlling the driving of a part to be driven. A value input from an input part such as an operations panel is added to this correction value so as to determine a final correction value. Thus, according to Patent Document 1, in forming a color image on the transfer paper, by controlling the driving of the part to be driven based on the determined final correction value, it is possible to eliminate misregistration of each color with more certainty and to obtain a color image without misregistration of any color.
According to Patent Document 2 listed below, an image forming apparatus disclosed therein corrects image distortion or color misregistration of a color image with high accuracy with respect to each pixel of the entire image. Pattern data are generated so that test patterns are formed at predetermined positions on recording paper. Misregistration at feature points included in the test patterns are detected from image data obtained by reading the recording paper having an image formed thereon with the test patterns added thereto. After correcting a deviation or inclination at the time of reading by image reading means, misregistration of the feature points from where they are supposed to be output are detected. An operation value for correcting each pixel is calculated by referring to the detected misregistration of the feature points. An operation for eliminating misregistration of each pixel at the time of image formation is performed using the operation value.                [Patent Document 1] Japanese Laid-Open Patent Application No. 2002-244393        [Patent Document 2] Japanese Laid-Open Patent Application No. 2003-255626        
However, conventional methods of correcting misregistration have the following problems. That is, even when misregistration correction is performed at one occasion, misregistration remains until the next occasion and a lack of document information is caused if formed correction patterns are over-detected or under-detected so as to prevent correction (amount) of misregistration from being calculated. Further, an attempt of perform another misregistration correction prolongs time for misregistration correction so as to reduce printing operation time.